In a liquid crystal display device, a classification based on an operating mode of liquid crystal molecules includes a phase change (PC) mode, a twisted nematic (TN) mode, a super twisted nematic (STN) mode, an electrically controlled birefringence (ECB) mode, an optically compensated bend (OCB) mode, an in-plane switching (IPS) mode, a vertical alignment (VA) mode, a fringe field switching (FFS) mode and a field-induced photo-reactive alignment (FPA) mode. A classification based on a driving mode in the device includes a passive matrix (PM) and an active matrix (AM). The PM is further classified into static, multiplex and so forth, and the AM is classified into a thin film transistor (TFT), a metal insulator metal (MIM) and so forth. The TFT is further classified into amorphous silicon and polycrystal silicon. The latter is classified into a high temperature type and a low temperature type according to a production process. A classification based on a light source includes a reflective type utilizing natural light, a transmissive type utilizing backlight and a transflective type utilizing both the natural light and the backlight.
The liquid crystal display device includes a liquid crystal composition having a nematic phase. The composition has suitable characteristics. An AM device having good characteristics can be obtained by improving the characteristics of the composition. Table 1 below summarizes a relationship in two characteristics. The characteristics of the composition will be further described based on a commercially available AM device. A temperature range of the nematic phase relates to a temperature range in which the device can be used. A preferred maximum temperature of the nematic phase is about 70° C. or higher and a preferred minimum temperature of the nematic phase is about −10° C. or lower. Viscosity of the composition relates to a response time in the device. A short response time is preferred for displaying moving images on the device. A shorter response time even by one millisecond is desirable. Accordingly, a small viscosity in the composition is preferred. A small viscosity at a low temperature is further preferred.
TABLE 1Characteristics of Composition and AM DeviceNo.Characteristics of CompositionCharacteristics of AM Device1Wide temperature range of a Wide usable temperature range nematic phase 2 Small viscosity 1) Short response time 3 Suitable optical anisotropy Large contrast ratio 4 Large positive or negative Low threshold voltage and dielectric anisotropy small electric power consumption Large contrast ratio 5 Large specific resistance Large voltage holding ratio and large contrast ratio 6High stability to ultraviolet light Long service life and heat1) A composition can be injected into a liquid crystal display device in a shorter period of time.
An optical anisotropy of the composition relates to a contrast ratio in the device. In accordance with a mode of the device, a large optical anisotropy or a small optical anisotropy, more specifically, a suitable optical anisotropy is required. A product (Δn×d) of the optical anisotropy (Δn) of the composition and a cell gap (d) in the device is designed so as to maximize the contrast ratio. A suitable value of the product depends on a kind of the operating mode. The suitable value is in the range of about 0.30 micrometer to about 0.40 micrometer in a device having the VA mode, and in the range of about 0.20 micrometer to about 0.30 micrometer in a device having the IPS mode or the FFS mode. In the above cases, a composition having a large optical anisotropy is preferred for a device having a small cell gap. A large dielectric anisotropy in the composition contributes to a low threshold voltage, a small electric power consumption and a large contrast ratio in the device. Therefore, a large dielectric anisotropy is preferred. A large specific resistance in the composition contributes to a large voltage holding ratio and a large contrast ratio in a device. Accordingly, a composition having a large specific resistance at room temperature and also at a high temperature in an initial stage is preferred. A composition having a large specific resistance at room temperature and also at a high temperature even after the device has been used for a long period of time is preferred. Stability of the composition to ultraviolet light and heat relates to a service life of the device. In the case where the stability is high, the device has a long service life. Such characteristics are preferred for an AM device for use in a liquid crystal projector, a liquid crystal television and so forth.
In a polymer sustained alignment (PSA) mode liquid crystal display device, a liquid crystal composition containing a polymer is used. First, a composition to which a small amount of polymerizable compound is added is injected into a device. Next, the composition is irradiated with ultraviolet light while voltage is applied between substrates of the device. The polymerizable compound is polymerized to produce a polymer network structure in the composition. In the composition, alignment of liquid crystal molecules can be controlled, and therefore a response time of the device is shortened and image persistence is reduced. Such an effect of the polymer can be expected for the device having a mode such as a TN mode, an ECB mode, an OCB mode, an IPS mode, a VA mode, an FFS mode or an FPA mode.
A composition having a positive dielectric anisotropy is used for an AM device having the TN mode. A composition having a negative dielectric anisotropy is used for an AM device having the VA mode. A composition having a positive or negative dielectric anisotropy is used for an AM device having the IPS mode or the FFS mode. A composition having a positive or negative dielectric anisotropy is used for a polymer sustained alignment (PSA) mode AM device. Compound (1) in the present application is disclosed in Patent literature No. 1 described below.